1. Field of the Invention
The present invention relates generally to a system for managing entrance/exit of people, customers in shops, cars or automobiles in a parking area or the like. More particularly, the invention is concerned with an IC card device used in the system such as mentioned above which device is carried by a person or installed in a car for transmitting data previously registered therein in response to reception of an inquiry radio signal issued by an inquiring unit installed at an entrance/exit port or like place.
2. Description of Related Art
Heretofore, the above-mentioned type IC card device carried by people, installed in cars or other mobile objects is so designed as to send back the data registered previously (e.g. identification data) in response to reception of an a radio wave inquiry signal transmitted from an inquiring unit disposed in the vicinity of an entrance/exit port or gate. On the other hand, the data received by the inquiring unit is processed for thereby permitting or inhibiting passage of the person, car or other mobile object through the gate. With this system, management of the people or cars passing through the gate can be performed very easily and conveniently.
However, the IC card device known heretofore suffers from a problem that when there exist a plurality of IC card devices making access to a single inquiring unit, there takes place interference among the identification data signals transmitted from the IC card devices, which makes it impossible for the inquiring unit to recognize discriminatively the individual identification data from one another.
As one of the measures to cope with the problem mentioned above, it is described in JP-A-2-226392 to control the field intensity of the inquiry radio signal when the presence of plural IC card devices is detected. However, this proposal is not effective to suppress the interference when a plurality of IC card devices continue to stay for a given time at one place, for example, in front of a gate obstructing passage of people or cars. Further, there is disclosed in JP-A-1-201793 a method according to which the inquiring unit commands the IC card device to stop transmitting the identification data when the inquiring unit recognizes the identification data. Although this system is effective for coping with the aforementioned problem, difficulty is yet seen in that the internal structure of the IC card device becomes very complicated.